why is my New Fixed Asset tick box greyed out on PO line details?

I am configuring our FA module initially in test environment.  I have ticked all the purchasing boxes on the FA parameters form to allow creation and acquisition from purchasing, but no luck.  Our PO's are auto-created from requisitions, but I want the procurement team to review and tick this box on the PO prior to confirmation when required to allow for automatic asset creation upon receipt or invoicing.

The box is greyed out though whatever I do.  Anyone know why this is? (AX2012R2 CU7)

thanks